Previous studies have shown that adherence-defective mutants of Streptococcus mutans PS14, serotype c, can be grouped into several different phenotypic groups. In this study a method was developed to test for complementation between pairs of nonadhering mutants which possess different genotypic defects. Mutant strains UAB95 and a spectinomycin-resistant derivative of UAB95 (UAB516) were found to exhibit increased levels of adherence when grown together with UAB230 in media containing sucrose as compared to the adherence of each strain grown separately. An increase in caries was also observed in gnotobiotic rats mixedly infected with the two mutants as compared to either strain alone. Tests revealed that UAB95 produced more water-insoluble glucan than its parent strain but had a defect in glucan binding. UAB230 was found to produce levels of a defective glucan that could not be bound by mutant or wild-type cells. Our results suggest that UAB95 produces a water-insoluble glucan which is bound by UAB230, thus allowing complementation for adherence and caries production.